1. Field of the Invention
The present invention relates to a planographic printing plate precursor that has sensitivity to a UV ray, a visible ray, or an infrared ray. More specifically, the present invention relates to a so-called negative-type image recording material capable of directly plate-making by using a laser from a digital signal of a computer or the like.
2. Description of the Related Art
Due to the latest noticeable development of lasers, lasers which are capable of outputting high-power in any of a UV region, a visible region and an infrared region and made compact have been easily made available. Such laser are very useful as recording light sources in directly plate-making on the basis of digital data of a computer or the like (‘Computer to Plate’ which is abbreviated as ‘CTP’ hereinafter). A solid-state laser and a semiconductor laser which emit an infrared ray having a wavelength of 760 nm to 1200 nm are extremely useful because these lasers can output higher power than the lasers having other wavelength regions. Accordingly, in these years, a demand for such an image recording material which is highly sensitive to an infrared laser, i.e., an image recording material whose solubility in a developer varies largely when exposed with an infrared laser has increased.
Ordinarily, a plurality of planographic printing plate precursors are stacked, stored and conveyed. During this process, the front surface of the image recording layer are stuck to the rear surface of the support. In the negative-type image recording layer that is cured by light exposure, strength of the image recording layer prior to curing via cross-linking or polymerization is relatively low. Meanwhile, in the negative-type image recording layer that includes a low molecular polymerizable compound, the support and the image recording layer are easily stuck to each other. In both cases, a problem arises that, when the stack of the materials is dislocated or when the stacked materials are conveyed sequentially one plate after another from the top for processing, the image recording layers are likely to suffer scratches due to rubbing.
In a general purpose image writing method using visible light, an image forming reaction is effected by using light both in laser exposure and in lamp exposure to allow sufficient optical reaction to occur at a deeper portion in the image recording layer, i.e., in the vicinity of the support. Therefore, partly missing of the image of at an image area of the image recording layer (hereinafter, “image missing”) does not occur even when only the front surface of the image recording layer receives scratches. Accordingly, image defects such as image missing occurs only when scratches are serious enough to reach the deepest portion of the image recording layer.
In another image writing method using an infrared laser as conducted by the present invention, an infrared ray is converted to heat to cause a temperature rise in the image recording layer, whereby an image is formed. In this method, when a support in general use, such as aluminum is employed, heat diffuses into the support which has high thermal conductivity. Therefore, a sufficient temperature rise is not achieved at the deeper portion of the image recording layer, while a sufficient temperature rise to cause an efficient thermal reaction is obtained at the portions in the vicinity of the front surface of the image recording. In such an image forming mechanism, even if only the front surface of the image recording layer receives a slight scratch, curing reaction takes place, and defects are produced at a region to effect curing reaction near the front surface, which is also a region resistant to a developer, whereby image-missing is likely to occur. Accordingly, in the image recording method using an infrared ray as conducted by the present invention, an image forming material is more easily affected by such scratches to noticeably impair image quality than in the image recording method using a visible light.
The present inventors conducted extensive researches and clarified that a problem arises that rubbed portions remain as a residual layer at non-image areas of the image recording layer after completion of a development processing. This is presumably attributed to the facts that, when the front surface of the image recording layer and the rear surface of the support are rubbed together to create frictional heat, a radically polymerizable compound present in the image recording layer undergoes polymerization by the frictional heat, and cured portions of the image recording layer cannot be removed after the development, resulting in a residual layer produced at non-image areas.
Such a phenomenon is noticeable in the negative-type image recording material containing a light absorbent such as an infrared ray absorbent. It is supposed that, since the image recording layer has a mechanism by which an infrared ray is converted to heat to cause an image forming reaction, the image recording layer is more sensitive to heat created by friction, whereby the residual layer is more likely produced than that having another image forming mechanism.
In order to prevent the image recording layer from suffering scratches or prevent the residual layer from being produced at non-image areas of the image recording layer, it is designed to interpose protective paper (interleaf sheets) between stacked layers of the planographic printing plate precursors. However, when continuous plate-making of the planographic printing plate precursors is carried out, operations of peeling the interleaf sheets from the planographic printing plate precursors are required, to thus impair working efficiency.
Specifically in the aforementioned CTP outputting system, it is highly desired to reduce the cost of a device and increase the number of exposure plates per unit time. For this reason, it is useful to obviate a mechanism to remove interleaf sheets from a plate material feeder in the outputting system in order to simplify the device. That is, it is desired to feed materials in the form of non-interleaf sheets, and convey the same inside the outputting device.
Thus, there is a strong demand for a negative-type image recording material which is free from scratches or residual layer production even when the image recording materials are stacked and conveyed without interleaf sheets interposed therebetween.